The present invention relates to a liquid crystal display apparatus, and, especially, to an active matrix type liquid crystal display apparatus.
In the conventional active matrix type liquid crystal display apparatus, the method which employs nematic liquid crystals is used for all the distinctive liquid crystal display modes, such as the twisted nematic method and the horizontal electric field method. In the liquid crystal display apparatus using nematic liquid crystals, the liquid crystal responds to a voltage change with a relatively slow response time of from 15 msec to 50 msec for altering the display image from black to white or from white to black. The response time for altering the display image from white to a middle tone or from black to a middle tone is even slower, such as 40 msec to 150 msec, which may lead to after images, which appear like brushed pictures, in case of displaying motion pictures containing middle tone components, such as television pictures.
The display method in those conventional liquid crystal display apparatuses is called a “hold type” method in which an identical image is continuously presented during a single frame defined as a single cycle of the image signal.
In displaying motion pictures like television pictures with this hold type liquid crystal display apparatus, a moving object in a series of images to be animated continuously is displayed at a fixed position in a single frame. This means that the moving object is displayed at a proper position in a time slot within the single frame, but this moving object is displayed at an unexpected position and an unexpected image is displayed at the proper position at another time slot. The human sight recognizes those images as equalized images, which leads to fuzzy images.
As described above, there are two problems in displaying motion pictures using a liquid crystal display apparatus. As for the first problem, H. Okumura et al. “SID 92 DIGEST p.601 (1992)” and Japanese Patent Application Laid-Open No. 4-288589 (1992) disclose a technology in which the picture signal in the present frame supplied from the picture source is compared with the picture signal in the previous frame; and, in case any change in the picture signal is detected, the picture signal is emphasized and converted in order to enhance the change in the picture signal, whereby the display at the corresponding pixel is adjusted to a value corresponding to the desired picture signal until the next frame begins.
As for the second problem, K. Sueoka et al. “IDRC '97 PP.203 (1998)” discloses a technology in which the generation of fuzzy images due to the equalization operation is prevented by means whereby the liquid crystal is made to respond at first by scanning the whole liquid crystal panel, and next the illumination unit is turned on.
In the prior art described above in connection with the first problem, however, though the response with respect to the middle tone can be made faster by means of the image emphasis and conversion operation, since the display response for the individual pixel reaches a designated display result at the end of the single frame period (about 16.6 msec), there is still a problem in that the display result provided during this period may be recognized as after images.
In the prior art described above in connection with the second problem, however, since the illumination unit is turned on after the data has been written by scanning all the pixels in the liquid crystal display part and all the pixels respond completely, the scanning time and the response time of the liquid crystal should be required to be made extremely short. In addition, since the lighting time period of the illumination unit is short, its light intensity should be increased in order to establish a brightness equivalent to that in the prior art. For this reason, there is a problem in that the electric current supplied to the illumination unit increases and the lifetime of the illumination unit itself becomes shorter.
In attempting to combine advantageous aspects of the above-described known techniques, since it takes long time using the second prior art technique described above to scan all the pixels and write the data, the first prior art technique can not attain the required response time by itself, and thus, there is a problem in that the response time of the liquid crystal itself should be made much faster.
Otherwise, in case the first prior art technique is used for establishing enough response and then the second prior art technique is used for lighting the illumination unit, since the lighting time period of the illumination unit becomes extremely short, there is a problem in that the lifetime of the illumination unit becomes shorter because it is required to increase the amount of electric current to be supplied to the illumination unit.